Abstract
Cardiac imaging is a promising application for combined PET/MR imaging. However, current MR imaging protocols for whole‐body attenuation correction can produce spatial mismatch between PET and MR‐derived attenuation data owing to a disparity between the two modalities' imaging speeds. We assessed the feasibility of using a respiration‐averaged MR (AMR) method for attenuation correction of cardiac PET data in PET/MR images. First, to demonstrate the feasibility of motion imaging with MR, we used a 3T MR system and a two‐dimensional fast spoiled gradient‐recalled echo (SPGR) sequence to obtain AMR images of a moving phantom. Then, we used the same sequence to obtain AMR images of a patient's thorax under free‐breathing conditions. MR images were converted into PET attenuation maps using a three‐class tissue segmentation method with two sets of predetermined CT numbers, one calculated from the patient‐specific (PS) CT images and the other from a reference group (RG) containing 54 patient CT datasets. The MR‐derived attenuation images were then used for attenuation correction of the cardiac PET data, which were compared to the PET data corrected with average CT (ACT) images. In the myocardium, the voxel‐by‐voxel differences and the differences in mean slice activity between the AMR‐corrected PET data and the ACT‐corrected PET data were found to be small (less than 7%). The use of AMR‐derived attenuation images in place of ACT images for attenuation correction did not affect the summed stress score. These results demonstrate the feasibility of using the proposed SPGR‐based MR imaging protocol to obtain patient AMR images and using those images for cardiac PET attenuation correction. Additional studies with more clinical data are warranted to further evaluate the method.PACS number: 87.57.uk
Highlights
312 Ai et al.: Averaged MR for cardiac positron emission tomography (PET)/MR attenuation correction over several minutes, each CT slice is captured in less than 1 s
For cardiac PET/CT attenuation correction, the use of respiration-averaged CT (ACT) images has been reported to reduce respiratory motion-induced misalignment of PET and CT images.[13,16,17] we posit that using respiration-averaged MR (AMR) images for attenuation correction could reduce misalignment between cardiac PET and MR data and reduce myocardial perfusion artifacts in PET/MR images
AMR images of the phantom and patient For both the phantom under simulated respiratory motion and the patient under free-breathing conditions, visual inspection of the acquired MR images revealed that the proposed MR protocol could generate AMR images without visible motion artifacts and with average motion blurring effect, which is crucial to the success of the proposed technique (Fig. 2)
Summary
312 Ai et al.: Averaged MR for cardiac PET/MR attenuation correction over several minutes, each CT slice is captured in less than 1 s. In whole-body PET/ MR imaging, MR images for attenuation correction, unlike PET data, are usually acquired using a breath-hold Dixon sequence, which takes about 18 s for each 21 cm bed position.[14] Examples of respiration associated attenuation artifacts in clinical whole-body PET/MR have been reported by Keller et al[15] The difference in image acquisition time suggests that artifacts caused by spatial mismatch can occur in cardiac PET/MR imaging. As a proof of concept, in the present study, we: 1) proposed a spoiled gradient-recalled echo (SPGR)-based MR imaging protocol for obtaining cardiac AMR images under free-breathing conditions; 2) demonstrated the feasibility of deriving attenuation maps from AMR data; and 3) evaluated the proposed technique in a patient study
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